1. Field of the Invention
This invention is directed to an electrophoretic display device and a method for operating the same. More specifically, the electrophoretic display device includes unit cells, each of which includes transmissive particles and a reflective panel, wherein the unit cells are vertically laminated or arranged in parallel, thereby achieving high reliability, high reflectance and improved color characteristics.
2. Description of the Related Art
Electrophoresis is a phenomenon in which charged particles migrate when an electric field is applied to a dispersion of the particles in a fluid. Electrophoretic display devices refer to image display apparatuses based on the electrophoresis which enable repeated writing and erasing of shapes and letters.
FIGS. 1a and 1b are schematic cross-sectional diagrams illustrating the structure and operation of a conventional electrophoretic display device. Referring to FIGS. 1a and 1b, charged white particles 130 and charged black particles 140 are introduced between two panels 110 and 120, each of which has a conductive electrode. The charged particles 130 and 140 migrate to the different panels 110 and 120 under the influence of an applied electric field because they are oppositely charged in polarity. As shown in FIG. 1a, when the white particles 130 are negatively charged and the black particles 140 are positively charged, the black particles 140 are adsorbed to the upper panel 110 to which a negative potential is applied and the white particles 130 are adsorbed to the lower panel 120 to which a positive potential is applied. The black particles 140 completely absorb white light incident from an external source and reflect no light, so that an observer perceives a black color. Conversely, when a positive potential is applied to the upper panel 110, the white particles 130 are adsorbed to the upper panel 110 to completely reflect the incident light (see FIG. 1b), thus allowing the observer to perceive a white color. A color display device can be fabricated in accordance with the same procedure as described above except that colored particles are introduced instead of black particles and white particles.
However, the use of the two types of oppositely charged particles in the conventional display device may cause poor reliability and difficulties in operating the display device. Specifically, the two types of oppositely charged particles are brought into contact with each other due to their electric attraction (see FIG. 2a) and are finally neutralized. The neutralized particles lose their electrostatic properties, and as a result, they do not respond any more to the applied electric field.
The charged particles may be surrounded with insulating films or processed so as not to exchange electrons. In any case, however, since electric attractive forces act between the particles, the two types of particles are brought into contact with each other to form dipoles. Once the dipoles are formed, an electric field for operation must be additionally applied to destroy the dipoles and move the particles, resulting in an increase in operation voltage (see FIG. 2b). Further, since the display device employs a scattering mode for operation, the colored particles are required to play a role in scattering light. It is desirable that the particles be several micrometers in diameter to increase the scattering efficiency of light. The use of large amounts of the large-size particles inevitably leads to an increase in operation voltage and a delay in response time. To the contrary, a decrease in the size of the particles leads to a reduction in the reflectance of the display device.